Each edit shifted every later line start in the line index, O(lines) per
keystroke (0.5 ms near the top of a 1M-line file). The shift is now kept
pending ("from entry k, add d") and applied on read; further edits at the
same line only adjust it, and it is folded in when an edit lands
elsewhere.
locate() walked the piece list for every lookup (two per rendered line
per frame on an edited buffer); it now binary-searches prefix sums
rebuilt once per modification (keyed on version_, which every mutation
bumps; AdoptOriginal now bumps it after installing its piece).
Typing on line 10 of a 1M-line file with a frame's worth of line reads
per keystroke: 0.52 ms -> 0.017 ms. A new randomized test checks every
read path against a string model with consolidation forced; it fails
under each of three deliberate breakages of the new bookkeeping.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB
Claude
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